Sintered-bonded high temperature coatings for ceramic turbomachine components

ABSTRACT

Methods for forming sintered-bonded high temperature coatings over ceramic turbomachine components are provided, as are ceramic turbomachine components having such high temperature coatings formed thereover. In one embodiment, the method includes the step or process of removing a surface oxide layer from the ceramic component body of a turbomachine component to expose a treated surface of the ceramic component body. A first layer of coating precursor material, which has a solids content composed predominately of at least one rare earth silicate by weight percentage, is applied to the treated surface. The first layer of the coating precursor material is then heat treated to sinter the solids content and form a first sintered coating layer bonded to the treated surface. The steps of applying and sintering the coating precursor may be repeated, as desired, to build a sintered coating body to a desired thickness over the ceramic component body.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/608,574 filed on May 30, 2017. The relevant disclosure of the aboveapplication is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to turbomachine components and,more particularly, to sintered-bonded high temperature coatings formedover selected surfaces of ceramic turbomachine components, as well as tomethods for forming such high temperature coatings.

ACRONYMS AND ABBREVIATIONS

The following acronyms appear throughout this document:

CMAS—Calcium-Magnesium Aluminosilicates;

CTE—Coefficient of Thermal Expansion;

EBC—Environmental Barrier Coating;

GTE—Gas Turbine Engine;

HP—High Pressure;

SEM—Scanning Electron Microscope;

TBC—Thermal Barrier Coating;

vol %—volume percentage; and

wt %—weight percentage.

BACKGROUND

Fuel efficiency, emission levels, thrust-to-weight ratios, and othermeasures of GTE performance can be boosted by increasing the core gastemperatures at which the GTE operates. To support high temperature GTEoperation, GTE components located within the core gas flow path arecommonly fabricated from superalloy materials, which have enhancedproperties at elevated temperatures. GTE components composed ofsuperalloy materials (herein, “superalloy GTE components”) may stillimpose undesired temperature limitations on GTE operation, however,particularly in the case of emerging GTE platforms designed to operateat ever-increasing core gas temperatures. The temperature capabilitiesof superalloy GTE components can be improved through the usage of activecooling techniques and the provision of TBCs; however, such approachesprovide only limited enhancements in temperature capabilities and addundesired cost and complexity to component manufacture. For thesereasons, industry attention is increasing focusing on the fabrication ofGTE components from advanced ceramic materials, which can withstandprolonged exposure to operative temperatures higher than those tolerableby comparable superalloy materials. Ceramic materials are also typicallyless dense than their superalloy counterparts and, thus, may alsoprovide weight savings for flight applications.

While providing the above-noted benefits, GTE components composed ofceramic materials (herein, “ceramic GTE components”) remain limited. Asone primary limitation, ceramic GTE components are often prone toundesirable high temperature reactions with combustive byproducts. As aspecific example, GTE components fabricated from silicon-ceramicmaterials, such as silicon carbide and silicon nitride, may besusceptible to recession due to high temperature reactions with mixturesof water vapor and oxygen (colloquially, “steam”). The silicon containedin such materials readily oxides to form silica, which reacts with steamat elevated temperatures to form volatile silicon hydroxide. Sublimationof the silicon hydroxide may then accelerate erosion of thesilicon-ceramic material and drive rapid recession of the componentbody. EBCs can be formed over ceramic GTE components to provide enhancedprotection from such high temperature reactions. Conventional EBCs,however, are also often susceptible to high temperature steampenetration and typically rely on metallic bond coats to join the EBC tothe underlying component body. In many cases, the metallic bond coat mayitself contain silicon and, thus, may also be prone to structuraldegradation due to high temperature steam reactions. In certaininstances, fractures occurring within the bond coat and along the bondcoat interfaces can result in premature EBC spallation and failure. As astill further limitation, conventional EBCs are typically poor thermalinsulators and do little to shield the underlying ceramic component bodyfrom elevated surface temperatures.

There thus exists an ongoing demand for protective high temperaturecoatings suitable for formation over ceramic GTE components, whichovercome one or more the limitations set-forth above. Ideally, such hightemperature coatings would provide both thermal and environmentalbarrier protection, including resistance to high temperature steampenetration. It would also be desirable for such high temperaturecoatings to be relatively resistant to spallation and similar structuralcompromise within the high temperature GTE environment. Processes forforming such high temperature coatings over ceramic GTE components and,more generally, over ceramic turbomachine components are providedherein, as are ceramic turbomachine components having surfaces protectedby high temperature coatings. Other desirable features andcharacteristics of embodiments of the present invention will becomeapparent from the subsequent Detailed Description and the appendedClaims, taken in conjunction with the accompanying drawings and theforegoing Background.

BRIEF SUMMARY

Methods for forming sintered-bonded high temperature coatings overceramic turbomachine components, such as silicon-ceramic GTE components,are provided. In various embodiments, the method includes the step orprocess of removing a surface oxide layer from the ceramic componentbody of a turbomachine component to expose a treated surface of theceramic component body. A first layer of coating precursor material,which has a solids content composed predominately of at least one rareearth silicate by weight percentage, is applied to the treated surface.The first layer of the coating precursor material is heat treated tosinter the solids content and form a first sintered coating layer bondedto the treated surface. The steps of applying and sintering the coatingprecursor may be repeated, as desired, to successively build or compilea sintered coating body to a desired thickness over the ceramiccomponent body. In certain implementations, the sintered coating bodymay be imparted with a controlled porosity by, for example, selectivelyembedding fugacious organic particles in one or more layers of thecoating precursor material and thermally decomposing the organicparticles during subsequent heat treatment. In such embodiments, thecontrolled porosity is usefully, but not necessarily varied by design,as taken through the thickness of the sintered coating body.

In other embodiments, the high temperature coating fabrication methodincludes the step of process of successively compiling or building-up asintered coating body over a selected surface of a ceramic componentbody, such as the body of a silicon-ceramic GTE component. The sinteredcoating body may be compiled or built-up over the ceramic component bodyby repeating alternating the steps of: (i) depositing a coatingprecursor material containing rare earth silicate particles over theselected surface of the ceramic component body, and (ii) heat treatingthe coating precursor material to sinter the rare earth silicateparticles and form one or more sintered coating layers. Fugaciousorganic particles are selectively added to the coating precursormaterial and thermally decomposed during heat treatment to impart thesintered coating body with a varied porosity, as taken along an axisorthogonal to the selected surface. In certain embodiments of the hightemperature coating fabrication method, a surface oxide may be etchedaway or otherwise removed from the selected surface of the ceramiccomponent body prior to successively compiling the sintered coating bodythereover. Additionally, steps may be taken to substantially preventsurface oxide regrowth prior to formation of at least an innermostportion of the sintered coating body (that is, the portion of thecoating body closest the ceramic component body) such that the innermostportion of the sintered coating body is bonded directly to andintimately contacts the selected surface of the ceramic component body.

Embodiments of coated turbomachine components are further provided. Invarious embodiments, the coated turbomachine component includes aceramic component body having a principal surface, as well as a hightemperature coating comprising a sintered coating body bonded directlyto and intimately contacting the principal surface of the ceramiccomponent body. The sintered coating body has a minimum porosityadjacent the principal surface and a maximum porosity at a locationfurther from the principal surface, as taken along an axis orthogonal tothe principal surface. In certain implementations, one or more of thefollowing conditions may further apply: (i) the minimum porosity is lessthan 10% by volume, while the maximum porosity is between 20% and 40% byvolume; (ii) the sintered coating body may contain about 80% to 100% and0% to about 20% of at least one glass sintering aid by weight; and/or(iii) the principal surface of the ceramic component body may besubstantially free of oxide contaminants and may have an average surfaceroughness exceeding 01. microns (μm) and possibly exceeding 0.5 μm incertain instances. Finally, in one specific and non-limitingimplementation, the ceramic component body assumes the form of a turbineshroud body, which is composed of a silicon-ceramic material and whichhas an inner peripheral surface over which the high temperature coatingis formed.

BRIEF DESCRIPTION OF THE DRAWINGS

At least one example of the present invention will hereinafter bedescribed in conjunction with the following figures, wherein likenumerals denote like elements, and:

FIG. 1 is an isometric view of a GTE (partially shown), which contains aceramic turbine rotor shroud over which a sinter-bonded high temperaturecoating is usefully formed, as illustrated in accordance with anexemplary embodiment of the present disclosure;

FIG. 2 is a cross-sectional schematic of a magnified region of theceramic turbine rotor shroud shown in FIG. 1, which illustrates onepossible implementation of the sinter-bonded high temperature coating ingreater detail;

FIG. 3 is a flowchart setting-forth an exemplary process for forming asinter-bonded high temperature coating over a ceramic turbomachinecomponent, such as the ceramic turbine rotor shroud shown in FIGS. 1-2;

FIGS. 4-7 illustrate a generalized ceramic turbomachine component andhigh temperature coating during various sequentially-performed stages ofthe exemplary high temperature coating formation process of FIG. 3; and

FIG. 8 is an SEM image of a sinter-bonded high temperature coating, asreduced to practice and further produced in accordance with theexemplary high temperature coating formation process set-forth in FIG.3.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the exemplary and non-limiting embodimentsdescribed in the subsequent Detailed Description. It should further beunderstood that features or elements appearing in the accompanyingfigures are not necessarily drawn to scale unless otherwise stated. Forexample, the dimensions of certain elements or regions in the figuresmay be exaggerated relative to other elements or regions to improveunderstanding of embodiments of the spallation-resistant hightemperature and coated turbomachine components described herein.

DETAILED DESCRIPTION

The following Detailed Description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by any theorypresented in the preceding Background or the following DetailedDescription. The term “exemplary,” as appearing throughout thisdocument, is synonymous with the term “example” and is utilizedrepeatedly below to emphasize that the following description providesonly multiple non-limiting examples of the invention and should not beconstrued to restrict the scope of the invention, as set-out in theClaims, in any respect. As further appearing herein, statementsindicating that a first layer is “bonded to” or “joined to” a secondlayer, surface, or body do not require that that the first layer isdirectly bonded to and intimately contact the second layer, surface, orbody unless otherwise specifically stated.

OVERVIEW

The following describes processes for forming high temperature coatingsover selected surfaces of ceramic turbomachine components, such GTE andturbocharger components. The high temperature coatings described hereinmay provide both thermal barrier protection and environment protectionto silicon-ceramic GTE components, which are directly exposed to coregas flow during GTE operation; however, embodiments of the hightemperature coating are not restricted to usage in conjunction with anyparticular type of GTE component. The following also describes coatedturbomachine components having ceramic component bodies (also referredto below as “substrates”) over which high temperature coatings areformed. The high temperature coatings described herein may be fabricatedutilizing unique sintering processes and, when so formed, may bereferred to as “sintered-bonded high temperature coatings.”

Embodiments of the high temperature coating is are bonded directly tothe underlying ceramic component body, thus eliminating reliance on anintervening bond coat for attachment purposes. Processing steps,requisite materials, and manufacturing costs may be favorably reducedthrough the elimination of any such bond coat. Furthermore, failurepaths commonly observed in bond coat-reliant coating systems andstemming from bond coat structural compromise may be mitigated. In theabsence of the bond coat, an intimate and mechanically-robust bond maybe created between the high temperature coating and the underlyingceramic component body utilizing a sinter bonding process. Prior tosintering bonding, surface oxides present on selected surfaces of theceramic component body may be removed. Additional measures may also betaken to deter surface oxide regrowth until coating sinter bonding is atleast partially completed. Through such steps, a highly robustmechanical bond can be formed between the intimately-joined hightemperature coating and the underlying ceramic component body. Thismechanical bond may be further enhanced through tailored coatingformulations and/or by imparting the component body surfaces with arelatively rough surface topology during oxide removal, as describedmore fully below.

The high temperature coating may provide an environmental barrierfunction by shielding the underlying component body from contaminants,such as combustive byproducts. When formed over a silicon-ceramiccomponent body, the high temperature coating may resist penetration ofhigh temperature mixtures of water vapor and oxygen (herein, “steam”),which could otherwise drive recession of the underlying silicon-ceramicmaterial by the mechanisms previously described. The high temperaturecoating may also serve as a thermal barrier due, at least in part, to acontrolled, non-zero porosity within the coating body. The porosity ofthe high temperature coating may further enhance strain compliance tobetter accommodate CTE mismatches between the coating and the underlyingcomponent body. In certain implementations, the porosity of the hightemperature coating may vary through the coating thickness, with minimumand maximum porosities potentially varying by a factor of two or more.By strategically varying coating porosity, the thermally insulativeproperties and strain compliance of the high temperature coating can beoptimized, while further ensuring that the coating remains relativelyresistant to steam penetration and is securely bonded to the underlyingcomponent body. The end result is a spallation-resistant, sinter-bondedhigh temperature coating, which can provide both environmental andthermal barrier protection for silicon-ceramic GTE components and otherceramic turbomachine components. This is highly desirable. An exemplaryembodiment of such a sinter-bonded high temperature coating will now bedescribed in conjunction with FIGS. 1-2.

Examples of Ceramic Turbomachine Components Including Sinter-Bonded HighTemperature Coatings

FIG. 1 is a cross-sectional schematic of a GTE 20 (partially shown)including a ceramic turbine rotor shroud 22, as illustrated inaccordance with an exemplary embodiment of the present disclosure. Asshown in FIG. 1 and described below, ceramic turbine rotor shroud 22 isprovided as a representative, but non-limiting example of a ceramicturbomachine component over which a sinter-bonded, high temperaturecoating can be beneficially formed. The following descriptionnotwithstanding, the below-described high temperature coating can beformed over various other ceramic turbomachine components in furtherembodiments. For example, in the context of GTE platforms, thebelow-described high temperature coatings may be formed over gas-exposedsurfaces of other GTE components fabricated from ceramic materialsincluding combustor liners, turbine nozzles, duct members, compressorshrouds, compressor rotor blades, and turbine rotor blades, to list buta few examples.

In addition to turbine rotor shroud 22, the illustrated portion of GTE20 further includes a downstream or outlet end of a combustor 24, aturbine nozzle 26, an HP turbine rotor 28, and a surrounding engine case30. The aforementioned components are only partially shown in FIG. 1,but are each generally axisymmetric about the centerline or rotationalaxis 32 of GTE 20. A core gas flow path 34 extends through theillustrated portion of GTE 20 and is generally defined by combustor 24,turbine nozzle 26, HP turbine rotor 28, and turbine rotor shroud 22.During operation of GTE 20, combustive gasses are discharged fromcombustor 24 and progress along flow path 34. HP turbine nozzle 26 ispositioned downstream of combustor 24 and upstream of HP turbine rotor28. HP turbine nozzle 26 thus receives the hot, combustive gas flowdischarged by combustor 24 during GTE operation. HP turbine nozzle 26meters, accelerates, and turns the combustive gas flow toward blades 36of HP turbine rotor 28. The combustive gas flow drives rotation of HPturbine rotor 28 and the non-illustrated GTE shaft or shafts. This, inturn, drives the rotation of other non-illustrated GTE components (e.g.,a fan or compressor rotor) and provides power output, which may beextracted from GTE 20 in different forms. Turbine rotor shroud 22 helpsguide and contain the hot combustive gasses as they drive rotation of HPturbine rotor 28.

Turbine rotor shroud 22 is fabricated from a ceramic material, such as amonolithic or composite silicon-ceramic material of the type describedbelow. Fabrication of turbine rotor shroud 22 from a ceramic material(as opposed to a superalloy material) may favorably increase thetemperature capabilities of rotor shroud 22, which is exposed to peaktemperatures and gas flow velocities due to its positioning downstreamof combustor outlet end 24 and around HP turbine rotor 28. Fabricationof turbine rotor shroud 22 from a less dense ceramic material ratherthan a superalloy material may also provide weight savings, as valued inflight applications. These benefits notwithstanding, ceramic materialsare commonly prone to structural degradation, such as materialrecession, due to high temperature reactions with combustive byproducts,such as steam, as previously described. Therefore, to shield the ceramicbody of shroud 22 from undesired interactions with such combustivebyproducts, a high temperature coating 38 is formed over one or moregas-exposed surfaces of turbine rotor shroud 22, particularly the innerperipheral surface of shroud 22. High temperature coating 38 may furtherthermally insulate the shroud component body from the peak localtemperatures occurring at the gas-exposed surfaces of coating 38. Stillfurther desirable characteristics may be provided by high temperaturecoating 38, as will become apparent from the following description.

FIG. 2 is a cross-sectional view of a magnified region of turbine rotorshroud 22 illustrating a representative portion of high temperaturecoating 38 in greater detail, as shown in accordance with an exemplaryembodiment of the present disclosure. Here, high temperature coating 38is presented as a generalized schematic and is not drawn to scale;hence, the depicted coating layers contain within high temperaturecoating 38 and described below may vary in relative thickness in actualimplementations of coating 38. In addition to high temperature coating38, the illustrated portion of turbine rotor shroud 22 includes aceramic component body 46 (partially shown) having a principal surface48 over which high temperature coating 38 is formed. Principal surface48 may correspond to the inner peripheral surface of turbine rotorshroud 22 shown in FIG. 1, which faces the high temperature gas flowconducted through GTE 20 (FIG. 1). Component body 46 serves as the basestructure over which high temperature coating 38 is formed and isconsequently be referred to hereafter as “ceramic componentbody/substrate 46.”

As appearing herein, the terms “ceramic component body” and “ceramicsubstrate” are utilized interchangeable to refer to a body, substrate,or structure composed predominately of one or more ceramic materials, bywt % and/or vol %. The term “ceramic,” in turn, refers to an inorganicand non-metallic material, whether crystalline or amorphous instructure. The term “ceramic” is defined to encompass both monolithicand composite materials. Finally, the term “silicon-ceramic material”refers to a ceramic material containing silicon as a primary constituentby wt % and/or vol %. Silicon-ceramic materials suitable for producingthe ceramic component bodies and substrates described herein (e.g.,ceramic component body/substrate 46 shown in FIG. 2) include monolithicceramic materials, such as silicon carbide (SiC) and silicon nitride(Si₃N₄), and composite ceramic materials, such as siliconcarbide-silicon carbide (SiC/SiC) composites and other siliconcarbide-based composites.

High temperature coating 38 contains a sintered coating body 50. In manycases, sintered coating body 50 may make-up or constitute the volumetricmajority of high temperature coating 38; and, in certain instances,sintered coating body 50 may constitute the entirety of high temperaturecoating 38. Sintered coating body 50 is, in turn, comprised of multiplesintered coating layers, which are successively formed over ceramiccomponent body/substrate 46 to successively build-up sintered coatingbody 50 to a desired thickness. In depicted embodiment, four suchsintered coating layers are shown and identified by reference numerals50(a)-(d). In the following description, sintered coating layer 50(a) isreferred to as the “innermost” or “base” sintered coating layer; theseterms indicating that layer 50(a) is located closest to ceramiccomponent body/substrate 46, as taken through the coating thicknessalong an axis orthogonal to principal surface 48 (corresponding to theY-axis identified by coordinate legend 52 in FIG. 2). Conversely,sintered coating layer 50(d) is referred to below as the outermost layercontained in sintered coating body 50; the term “outermost” indicatingthat layer 50(d) is located furthest from ceramic componentbody/substrate 46 relative to coating layers 50(a)-(c), as taken throughthe coating thickness. While four sintered coating layers are shown inFIG. 2, high temperature coating 38 may include fewer or a greaternumber of sintered coating layers in further embodiments.

The respective compositions of sintered coating layers 50(a)-(d) mayvary on a layer-to-layer basis in certain embodiments. In otherembodiments, relatively little, if any variance exists between therespective compositions of sintered coating layers 50(a)-(d), possiblyexcluding slight variations in additives between the coating layers. Insuch embodiments, sintered coating body 50 may be described aspossessing a substantially homogeneous or uniform chemical composition,as taken through its thickness. By way of example, at least one andperhaps all of sintered coating layers 50(a)-(d) may be predominatelycomposed of one or more rare earth silicates, by wt % and/or vol %. Suchrare earth silicates may be selected from the group consisting ofgadolinium (Gd), lanthanum (La), lutetium (Lu) neodymium (Nd), samarium(Sm), scandium (Sc), terbium (Tb), ytterbium (Yb), yttrium (Y), andcombinations thereof. In one specific, albeit non-limitingimplementation, at least one of sintered coating layers 50(a)-(d) and,perhaps, all of coating layers 50(a)-(d) are predominately composed ofytterbium disilicate (Yb₂Si₂O₇) by wt %. In other implementations, atleast one of sintered coating layers 50(a)-(d) may contain more thanabout 80 wt %, preferably more than about 90 wt %, and still morepreferably more than about 95 wt % ytterbium disilicate. This stated,sintered coating layers 50(a)-(d) are not required to contain rare earthsilicates in all embodiments. Sintered coating layers 50(a)-(d) may eachbe substantially devoid of organic materials; the term “substantiallydevoid,” as appearing herein, defined as containing less than 1 wt %organic materials.

Sintered coating layers 50(a)-(d) may contain various other constituentsin addition to one or more rare earth silicates. Other inorganic ceramicadditives may be utilized to fine tune desired properties of hightemperature coating 38. In embodiments, at least one and possibly all ofsintered coating layers 50(a)-(d) contains a glass sintering aid, suchas magnesia, alumina, and/or magnesioaluminosilicate particles. In thiscase, sintered coating layers 50(a)-(d) may each contain between about0.5 wt % and about 10 wt % of the sintering aid (e.g., magnesia,alumina, and/or magnesioaluminosilicate) and may potentially containlesser amounts of other additives, such as lithia, borate, and/or zincoxide. In addition to or in lieu of glass sintering aids, various otheradditives (e.g., strength-increasing fibers) may also be introduced intosintered coating layers 50(a)-(d) to enhance the desired properties ofhigh temperature coating 38. Finally, sintered coating layers 50(a)-(d)may also each contain trace amounts of organic residue remaining fromfugacious organic materials initially contained in the coating precursormaterials and thermally decomposed during heat treatment, as discussedmore fully below in conjunction with FIG. 3.

Sintered coating body 50 may be imparted with a controlled, non-zeroporosity. Generally stated, increased coating porosities may improvestrain compliance within sintered coating body 50 and, therefore, enablehigh temperature coating 38 to better withstand mechanical stressorsresulting from any CTE mismatch with ceramic component body/substrate46. This may be particularly advantageous when sintered coating body 50is directly bonded to ceramic component body/substrate 46 andconsequently lacks an intervening bond coat, which may otherwise providea CTE bridge between coating body 50 and body/substrate 46. Increasedcoating porosity also tends to reduce the overall heat transfercoefficient (k) of sintered coating body 50 and, thus, enhance thethermally insulative properties of coating 38. Conversely, increasedcoating porosity may adversely impact the mechanical strength of hightemperature coating 38, detract from bond strength ceramic componentbody/substrate 46, and/or render sintered coating body 50 moresusceptible to high temperature steam penetration. These competingfactors can be balanced, in embodiments, by imparting sintered coatingbody 50 with a controlled, non-zero porosity, which is purposefullyvaried as taken through the coating thickness, as described more fullybelow.

To impart sintered coating body 50 with a thickness-varied porosity,voids may be created within one or more of layers 50(a)-(d) by selectiveaddition and thermal decomposition of fugacious organic particles (poreformers) during the below-described heat treatment process. Differentschemes in varying the porosity through the thickness of sinteredcoating body 50 may be employed. In certain embodiments, innermostsintered coating layer 50(a) may have a decreased average porosityrelative to sintered coating layer 50(b) and, perhaps, relative to theaverage porosity taken through the remainder of sintered coating body50. In this manner, bond strength at the interface of sintered coatinglayer 50(a) and ceramic component body/substrate 46 may be optimized,while the overall porosity of sintered coating body 50 is increased forimproved strain compliance, fracture-resistance, and thermal insulation.In other embodiments, innermost coating layer 50(a) and outermostcoating layer 50(d) may both be imparted with a decreased averageporosity relative to intermediate coating layers 50(b)-(c) and/or theaverage porosity of sintered coating body 50. Such controlled variationsin coating layer porosity may advantageously reduce the susceptibilityof high temperature coating 38 to high temperature steam penetration,while still providing sintered coating body 50 with an increasedporosity cumulatively. As a still further possibility, the coatinglayers may alternate in bilayer stacks between lower porosity and higheraverage porosities; e.g., in such implementations, coating layers 50(a),50(c) may have a decreased porosity relative to coating layers 50(b),50(d).

The porosities within coating layers 50(a)-(d) may also vary in certaininstances. For example, it may be the case that sintered coating body 50has decreased porosities in regions corresponding to one or more of thedashed lines shown in FIG. 2, which demarcate, in a generalize sense,the boundaries between sintered coating layers 50(a)-(d). Sinteredcoating body 50 may have a decreased porosity at one or more of theseboundaries in embodiments wherein layers of the coating precursormaterial are applied in a wet state during the coating formation processdescribed below in conjunction with FIG. 3. In such embodiments, the wetstate coating material may seep or wick into the pores of an underlyingcoating layer such a relatively non-porous or less porous band iscreated with sintered coating body 50 generally correspond to theinterface between sintered coating layers. This may be advantageous inthat such less porous bands or striations may further help seal interiorportions of coating body 50 from high temperature steam penetration.

Innermost sintered coating layer 50(a) is advantageously bonded directlyto principal surface 48 of ceramic component body/substrate 46.Depending upon the composition of ceramic component body/substrate 46,and pursuant to surface treatment (e.g., oxide removal) steps describedbelow, surface 48 may be characterized by a relatively rough topologycharacterized by feature depths exceeding 1 micron (μm) on average.Surface oxide growth along surface 48 is usefully minimized to avoidcovering such surface features, which can be leveraged to form a highlyrobust mechanical bond between high temperature coating 38 and ceramiccomponent body/substrate 46. Moreover, in embodiments wherein principalsurface 48 is characterized by a surface topology having threedimensional undercutting or overhanging (see FIG. 6), a mechanical lockeffect may be created between innermost sintered coating layer 50(a) andsurface 48 further enhancing bond strength. To achieve these desirableeffects, any surface oxide present on surface 48 may be removed (e.g.,by treatment with a chemical etchant) and subsequent measures may betaken to deter oxide regrowth prior to formation of sintered coatinglayer 50(a). Further description in this regard is provided below inconjunction with FIGS. 3-6.

If desired, a topcoat 54 can be formed over outermost sintered coatinglayer 50(d) and sintered coating body 50. When present, topcoat 54 maybe utilized to backfill any exposed pores presented along the outersurface of sintered coating layer 50(d), which may be precision groundprior to the application of topcoat 54. Topcoat 54 may serve as an outersealant layer further resisting contaminant ingress, such as steampenetration, into sintered coating body 50 during usage of hightemperature coating 38. In this case, topcoat 54 may be formulated tohave a porosity less than that of sintered coating layer 50(d) and maybe applied utilizing any suitable deposition processes, such as asol-gel deposition process. In other embodiments, topcoat 54 may have aformulation similar to that of sintered coating layer 50(b), but with ahigher glass content (e.g., magnesia, alumina, and/ormagnesioaluminosilicate) by wt %. Topcoat 54 can also be formulated toprovide other high temperature functions, if desired, such as increasedCMAS resistance and/or thermal barrier protection. In still otherimplementations, high temperature coating 38 may lack topcoat 54 suchthat sintered coating layer 50(d) is directly gas-exposed when turbinerotor shroud 22 (FIG. 1) is placed with the high temperature GTEenvironment. Exemplary processes for producing a high temperaturecoating over a turbomachine component, such as high temperature coating38 formed on ceramic turbine rotor shroud 22, will now be described inconjunction with FIG. 3.

Exemplary High Temperature Coating Formation Processes

FIG. 3 is a flowchart setting-forth a process 60 for forming asinter-bonded high temperature coating over selected surfaces of aceramic turbomachine component, as illustrated in accordance with anexemplary embodiment of the present disclosure. For consistency with theforegoing description, high temperature coating formation process 60 isprimarily described below in conjunction with the production of hightemperature coating 38 over body/substrate 46 of turbine rotor shroud 22shown in FIG. 1. This notwithstanding, process 60 can further beemployed to produce sinter-bonded high temperature coatings over variousother types of ceramic turbomachine components, without limitation. Hightemperature coating formation process 60 includes a number ofsequentially-performed process steps (STEPS 62, 64, 66, 68, 70, 72).Depending upon the particular manner in which high temperature coatingformation process 60 is implemented, each step generically illustratedin FIG. 3 may entail a single process or multiple sub-processes.Furthermore, the steps illustrated in FIG. 3 and described below areprovided by way of non-limiting example only. In alternative embodimentsof high temperature coating formation process 60, additional processsteps may be performed, certain steps may be omitted, and/or theillustrated steps may be performed in alternative sequences.

High temperature coating formation process 60 commences at STEP 62 bypreparing selected surfaces of the ceramic body of a turbomachinecomponent, such as turbine rotor shroud 22 (FIG. 1). Surface preparationmay entail and possibly consist essentially of a surface oxide removalprocess when such surface oxides are present on the selected componentsurfaces. Surface oxides will often be present for multiple reasons.First, ceramic materials, particularly silicon-ceramic materials, aretypically highly prone to rapid oxide (e.g., silica) growth when exposedto ambient air. Second, material vendors or suppliers commonly grownoxide layers on ceramic material bodies to protect the underlyingceramic material body. While this is immaterial, if not beneficial inmany instances, such surface oxides are usefully (although notessentially) removed during STEP 62 of process 60. Various differentoxide remove techniques may be utilized for this purpose includingchemical stripping; mechanical removal techniques, such lapping,grinding, and polishing; and combinations thereof.

Etching is a preferred technique utilized to strip surface oxides duringSTEP 62 of process 60 (FIG. 3). Further emphasizing this point, FIG. 4provides a highly generalized depiction of wet etch that may be employedfor this purpose. As depicted, a ceramic component body/substrate 46having a surface oxide layer 74 may be contacted with a wet etchchemistry; reference numeral “46” carried-over from FIG. 2 forconsistency. The liquid etchant may be present as a pool or bath 76.During the wet etch process, ceramic component body/substrate 46 may bedipped or otherwise submerged in bath 76, as indicated in FIG. 3 byarrow 78. Agitation and/or elevated bath temperatures may accelerate theetch process. After a predetermination duration of time, ceramiccomponent body/substrate 46 is removed from bath 76, as indicated byarrow 80. This results in the removal of surface oxides from thoseregions of ceramic component body/substrate 46 contacted by the wetetchant, as generically depicted on the right side of FIG. 3. Thisprocess may be performed in vacuum or in a non-oxidizing (e.g.,reducing) atmosphere to prevent rapid regrowth of the surface oxide uponremoval of ceramic component body/substrate 46 from bath 76. Theparticular etchant chemistry utilized will vary in relation to the typeof surface oxide removed, the composition of ceramic componentbody/substrate 46, and other such factors. By way of example, a moltensodium hydroxide (NaOH) etch chemistry is well-suited for the removal ofsilica scale from a silicon-ceramic component bodies. In alternativeembodiments, other etch chemistries and/or a dry etch (e.g., plasmaetching or reactive ion etching) may be employed.

Other processes can be performed during STEP 62 of process 60 (FIG. 3)to further prepare the selected surface or surfaces for high temperaturecoating formation. In this regard, the selected components surfaces maybe intentionally roughed by mild abrasion, chemical treatment, or thelike. If performed, such dedicated roughening steps may be carried-outprior to removal of the surface oxide. It has been found, however, thata relatively rough surface topology (that is, a surface topologycharacterized by average feature heights or depths exceeding 0.1 μm and,perhaps, exceeding 0.5 μm) may be achieved without such additionalroughing steps when, for example, glass phases present within thecomponent body or substrate material are removed from along the treatedsurface(s). As a more specific example, in instances in which ceramiccomponent body/substrate 46 (FIGS. 2 and 4) is composed of asilicon-ceramic material, such as a silicon nitride (SiN), siliconcarbide (SiC), or a silicon nitride-based composite, treatment withmolten sodium hydroxide (NaOH) or another silica-selective etchantchemistry can remove glass phases from the silicon-ceramic material and,in so doing, create a relatively rough surface topology promotingenhanced bonding with the subsequently-formed high temperature coating.This may be more fully appreciated by comparing FIGS. 5-6 (describedbelow), which are SEM images of the surface topology of a samplesilicon-ceramic component body or substrate captured prior to (FIG. 5)and after (FIG. 6) wet etch removal of the surface oxide (shown in FIG.5).

As indicated above, removal of surface oxides utilizing anoxide-selective etchant chemistry may concurrently remove glass phasesfrom the ceramic material to yield a relatively rough or non-planarsurface topology at the treated component surfaces. An example of thissurface topology is presented in the SEM image of FIG. 6. As can beseen, the resultant surface topology is generally characterized byrod-like projections having average feature heights approaching orexceeding 1 μm, as taken along an axis orthogonal to the treatedsurface; e.g., corresponding to the Y-axis identified by coordinatelegend 52 in FIG. 2. Such rod-like projections may also have undercut oroverhang regions, which provide a mechanical lock effect when contactedand infiltrated by the innermost sintered coating layer of the hightemperature coating; e.g., sintered coating layer 50(a) of hightemperature coating 38 (FIG. 2). Providing that any surface oxideregrowth is restricted to less than the feature height prior toformation of the innermost sintered coating layer, this mechanical lockeffect can be leveraged to achieve excellent bond strength between thesintered coating body and ceramic component body/substrate 46 in theabsence of an intervening bond coat.

With continued reference to FIG. 3, high temperature coating formationprocess 60 continues with producing, purchasing, or otherwise obtaininga coating precursor material (STEP 64, FIG. 3). The coating precursormaterial contains particulate solids, such as rare earth silicates,glass sintering aids, and other such constituents, which arenon-fugacious in nature and remain in the final, post-sintering coatingcomposition. Particle size, particle shape, composition, anddistribution of the solids content within the coating precursor materialare precisely controlled during the coating precursor materialproduction and application steps to yield desired results. In variousembodiments, the glass sintering aid particles may vary from about 0 wt% to about 20 wt % of total solids content within the coating precursormaterial, while rare earth silicate particles constitute a greaterportion of, and perhaps the remainder of, the solids content within thecoating precursor material. Similarly, in one non-limiting embodiment,the solids content of the coating precursor material may consistessentially of about 0 wt % to about 20 wt % glass sintering aid andabout 80 wt % to about 100 wt % rare earth silicates. In otherimplementations, the coating precursor material may contain betweenabout 90 wt % and about 99 wt % ytterbium disilicate particles, and/orabout 1 wt % to about 10 wt % of one or more glass sintering aidesselected from the group consisting of magnesioaluminosilicate, magnesia,and alumina particles. In still further embodiments, the coatingprecursor material may contain about 1 wt % to about 5 wt % and,perhaps, about 2 wt % to about 4 wt % magnesioaluminosilicate particles.

As indicated above, the coating precursor material may contain rareearth silicate particles (e.g., monosilicate or disilicate particles)and glass sintering aid particles of varying sizes and/or shapes.Suitable particle shapes include, but are not limited to, spherical,oblong, rod- or whisker-like, and platelet or laminae shapes. Inembodiments, the rare earth silicate particles may have a first averageminimum cross-sectional dimension (e.g., diameter in the case ofspherical particles) and may be combined with lesser amounts (by wt %)of one or more glass sintering aids. The glass sintering aid(s) mayhave, in turn, a second average minimum cross-sectional dimension (e.g.,diameter) greater than the first average minimum cross-sectionaldimension (e.g., diameter). As a more specific example, the rare earthsilicate (e.g., ytterbium disilicate) particles may have an averageminimum cross-sectional dimension (e.g. diameter) between about 1 μm andabout 5 μm, inclusive, while the sintering aid (e.g., alumina, magnesia,and/or magnesioaluminosilicate) particles may have a greater averageminimum cross-sectional dimension ranging between about 3 μm and about10 μm, inclusive. Various other additives may be introduced into thesolids content, as desired, including fibers or particles added forreinforcement purposes.

The non-fugacious solids content may be mixed or otherwise combined withfugacious or sacrificial organic ingredients, such as solvents, binders,surfactants, and other such chemicals, to produce the coating precursormaterial. Depending upon relative quantities, chemical types, andviscosities, the coating precursor material may be applied in a drystate or wet state as, for example, a solution, a suspension, a paste, atape, a slip, or a slurry, to list but a few examples. When a wet statedeposition technique is utilized, the coating precursor material mayhave a liquids content including a binder, such as isobutylmethacrylate; a solvent, such as a-terpineol; and/or a surfactant, suchas tridecyl acid phosphate or an alkyl phenol ethoxylated phosphoricacid sodium salt. In such embodiments, the binder may vary from about 0vol % to about 50 vol % of total liquid contents in the coatingprecursor material; the surfactant may vary from about 0 vol % to about10 vol % of total liquids content; and the solvent may vary from about 0vol % to about 100 vol % of total liquid content. In other embodiments,the liquid content of the coating precursor material may vary or thecoating material may be applied in a dry state (e.g., utilizing a thinfilm transfer process) and contain relatively little liquid or moisturecontent.

One or more layers of the coating precursor material are next depositedover the selected component surfaces at STEP 66 of high temperaturecoating formation process 60. Various different deposition techniquesmay be utilized, with preferred deposition techniques allowingapplication of relatively thin coating precursor layers (e.g., layershaving thicknesses less than 50 μm) having a substantially homogenousdistributions of the above-described coating constituents. Anon-exhaustive list of suitable deposition techniques includes painting,rolling, taping, screen printing, doctor blading, spin-coating,spray-coating, suspension/slip casting, dip-coating, and dry filmtransfer processes. Additional deposition steps may also be performed(that is, STEP 66 may be performed iteratively) following intermittentheat treatment steps, as described more fully below in conjunction withFIG. 7.

After deposition of the coating precursor material layer(s) (STEP 68,FIG. 3), a heat treatment or firing process is next performed (STEP 70,FIG. 3). Heat treatment may be conducted in multiple stages or phasescarried-out utilizing any number of furnaces, ovens, or other heattreatment systems. In various embodiments, an initial burnout phase isfirst conducted to thermally decompose any organic materials containedwithin the coating precursor materials. The organic burnout phase may becarried-out in accordance with a relatively gradual heating schedule;e.g., the coating precursor layers may be exposed to a temperature,which gradually increases to a peak process temperature of about 200degrees Celsius (° C.) to about 600° C. and is then maintained at thepeak process temperature for a predetermined time period of, forexample, several minutes to several hours, depending upon layerthickness, the quantity and type of fugacious particles contained withinthe coating precursor material (if present), and other such factors.Organic burnout may be carried-out in an air environment in certainembodiments. Alternatively, organic burnout may be performed in a vacuumor in a non-oxidizing atmosphere (e.g., a carbon monoxide, carbondioxide, or forming gas atmosphere) to deter surface oxide regrowth.

A higher temperature, rapid heating phase is further performed duringSTEP 70 of process 60 (FIG. 3) to sinter and densify the coatingprecursor material and thereby transform the coating precursor materialinto a sintered coating layer. The sintering phase of heat treatmentalso create the desired bond attaching and, perhaps, directly joiningthe innermost sintered coating layer (e.g., layer 50(a) in FIG. 2) tothe ceramic component body (e.g., ceramic component body/substrate 46 inFIG. 2). Peak temperatures during the sintering phase of heat treatmentmay range between about 1400° C. and about 1800° C. Thesintering/coating densification phase may produce optimal results whenfollowing a relatively aggressive heating schedule or temperatureramp-up; e.g., when heating the coating precursor material to the peakprocessing temperature, an average time-versus-temperature slope greaterthan about 300° C. per minute and, perhaps, greater than 375° C. perminute may be employed. In one implementation, the processingtemperature may increased from approximately room temperature (˜23° C.)to greater than 1500° C. in less than 4 minutes. Again, such a firing orsintering step may be carried-out in a non-oxidizing atmosphere or undervacuum to prevent or at least deter surface oxide regrowth.

Embodiments of the heat treatment process may also be described asentailing: (i) an organic burnout phase during which the first layer ofcoating precursor material is heated to a first peak temperature withina first time period, and (ii) a subsequently-performed sintering orfiring phase during which the first layer of coating precursor materialis heated to a second peak temperature within a second time period. Thefirst peak temperature may be less than the second peak temperature,while the first time period is greater than the second time period.Additionally, during the sintering phase, heating the first layer ofcoating precursor material at a rate exceeding 300° C. per minute toattain the second peak temperature.

As indicated in FIG. 3 at STEP 70, the foregoing process steps (STEPS66, 68) may be repeated to iteratively form additional sintered coatinglayers (e.g., coating layers 50(b)-(d) in FIG. 2). In this manner, thesintered coating body may be successively compiled or built-up over theceramic component body/substrate to a desired thickness. The manner inwhich coating precursor material deposition (STEP 66) and heat treatment(STEP 68) may be repeatedly and alternatively performed to compile thesintered coating body is further generically illustrated in FIG. 7.Specifically, FIG. 7 illustrates a series of sequentially-performeddeposition steps 92, 94, 96, which are interspersed with three heattreatment steps 98, 100, 102 to yield a sintered coating body 50 havinga desired thickness; reference numeral “50” carried-over from FIG. 2 forconsistency. In this example, a first series of deposition steps 92 isperformed, followed by a first heat treatment step 98, followed by asecond series of deposition steps 94, followed by a second heattreatment step 100, followed by a third series of deposition steps 96,and lastly followed by a third and final heat treatment step 102. Thenumber of coating deposition steps and sintering steps will vary amongstembodiments based, at least in part, on the desired final thickness ofsintered coating body 50 (and noting that some thickness may be removedfrom the sintered coating body via a subsequently-performed thinningstep, such as grinding or polishing). In one embodiment, the finalthickness of sintered coating body 50 may range from about 100 μm toabout 400 μm in an embodiment. In other embodiments, sintered coatingbody 50 may be thicker or thinner than the aforementioned range.

As noted above, sintered coating body 50 may be produced to containstriations or bands of decreased porosity, which extend along sinteredlayer-to-sintered layer boundaries or interfaces within body 50. Suchdense bands within body 50 may be created when layers of the coatingprecursor material are applied in a wet state during the coatingformation process; the wet-state coating precursor material seeps orwicks into an underlying, previously-formed, porous sintered coatinglayer to at least partially fill the pores exposed thereof; and a bandof decreased (possibly zero) porosity is created upon subsequent firingof the wet-state coating precursor material. This effect usefullyincrease the resistance of sintered coating body 50 to high temperaturesteam penetration in the GTE environment. If desired, theinitially-deposited layer of wet-state coating precursor material may beapplied in a state lacking organic fugacious particles (pore formers) orcontain a decreased amount of organic particles to enhance thisbeneficial effect. More specifically, in an implementation of theprocess set-forth in FIG. 7 in which the layers of coating precursormaterial are screen printed or otherwise deposited in a wet state, thelayer of coating precursor material initially laid down to begin eachdeposition step 92, 94, 96 may be deposited in a state containing alesser amount of fugacious organic particles relative to thesubsequently-applied layers of coating precursor material andpotentially in a state lacking fugacious organic particles entirely.

Certain benefits may achieved by forming sintered coating body 50 tohave a controlled, non-zero porosity, which varies through the coatingthickness. In various embodiments, the desired controlled and perhapsthickness-varied porosity is created by selectively adding fugaciousorganic particles to the base coating precursor material. During theheat treatment process, the fugacious organic particles thermallydecompose to create voids or gas-filled pockets within the sinteredcoating body having dimensions approximately equivalent to those ofremoved organic particles. The particular organic material or materialschosen for this purpose, the particle size or sizes, and the particleshape will vary amongst embodiments. In one embodiment, the fugaciousorganic particles are composed of a polymer, such as an acrylic. Thefugacious organic particles may have a spherical, oblong, rod-like, orlaminae shapes, or a combination thereof. In certain embodiments, theaverage minimal cross-sectional dimension of the particles (the averagediameter in the case of spherical particles) may range from about 1 μmto about 10 μm and, perhaps, from about 3 μm to about 6 μm. The peaktemperature utilized during heat treatment to remove the fugaciousspheres may range from about 450° C. to about 550° C. inimplementations. In other embodiments, the relevant parameters may begreater than or less than the aforementioned ranges.

The quantity of fugacious organic particles present in the coatingprecursor material can be varied between each series of deposition steps92, 94, 96 to create a controlled and varied porosity through thethickness of sintered coating body 50 (FIG. 2). In certain embodiments,the initially-applied layer or layers of coating precursor material(e.g., those layers applied at step 92 in FIG. 7) may contain a firstamount of organic particles, while one or more subsequently-appliedlayers of coating precursor material may have a second amount of organicparticles exceeding the first amount (which may be a zero value). Inthis manner, the resultant sintered coating body and, more generally,the resultant high temperature coating may be imparted with a first(e.g., minimum) porosity adjacent the component body/substrate and asecond (e.g., maximum) porosity further from the componentbody/substrate. The second (e.g., maximum) porosity may be at leasttwice the first porosity. For example, with reference to FIG. 2,fugacious organic particles may be added to the base coating precursormaterial after the first sintering step (and, therefore, after theinitial coating precursor material deposition) to impart sinteredcoating body 50 with a thickness-varied porosity, which increases in astep-wise or non-linear fashion when transitioning from innermostsintered coating layer 50(a) to overlying sintered coating layer 50(b),50(c), or 50(d). In alternative embodiments, the amount of organicparticles present in the coating precursor material may be varied toimpart with a porosity varying in another manner.

Process 60 concludes with STEP 72 (FIG. 3) during which final processingsteps are performed to complete fabrication of the high temperaturecoating. Such additional steps can include precision machining, theperformance of additional heat treatments steps, and/or the formation ofadditional coating layers over the sintered coating body produced duringSTEPS 66, 68, 70 (FIG. 3). An example of such an additional coatinglayer is topcoat 54 shown in FIG. 2. As previously discussed, such anadditional coating layer may be formed for enhanced sealing purposes incertain instances to, for example, impart the resultant high temperaturecoating with additional resistance to steam penetration. Such a sealanttopcoat can be applied as, for example, a sol-gel or by depositing andsintering a still further layer of the coating precursor material mixedwith an increased glass content. If formed, the sealant topcoat layer isadvantageously created after precision grinding of sintered coating body50. In other embodiments, a different topcoat layer or coating system(e.g., a TBC) may be formed over the sintered coating body (e.g.,sintered coating body 50 shown in FIG. 2 or sintered coating body 112shown in FIG. 7) or the high temperature coating may lack any additionalcoating layers.

Turning lastly to FIG. 8, there is shown a SEM image of a hightemperature coating 110 produced according with process 60 (FIG. 3) andreduced to practice. High temperature coating 110 is directly sinteredbonded to an underlying ceramic component body/substrate 118. In theillustrated example, high temperature coating 110 consists entirety of asintered coating body 112, which includes two sintered coating regionsor layers 114, 116. Sintered coating region 116 is the base or innermostcoating region and is thus located closer to ceramic componentbody/substrate 118 than is sintered coating region 114 (the outermostcoating region). Sintered coating region 114 overlies and is directlybonded to sintered coating region 116. Comparatively, sintered coatingregion 116 is bonded directly to and intimately contacts a selectedsurface of ceramic component body/substrate 118. In an embodiment,ceramic component body/substrate 118 may be composed of asilicon-ceramic material from which surface oxides have been removedutilizing a chemical etchant imparting principal surface 120 with arough or non-planar topology; e.g., surface 120 may be characterized bya topology having average feature height or depth exceeding 0.1 μm, aspreviously described.

As can be seen in the SEM image of FIG. 8, sintered coating region 114has an increased porosity relative to region 116. By way of example,sintered coating region 114 may have a first (maximum) average porosityranging from about 20 vol % to about 40 vol %, while sintered coatingregion 116 has a second (minimum) average porosity less than the first(maximum) average porosity and, perhaps, less than about 10 vol %. Thesedisparate porosities may be created utilizing the fugacious organicparticle approach described above in conjunction with FIG. 3. Thedecreased porosity of innermost sintered coating region 116 may improvebond integrity with ceramic component body/substrate 118 and enhance theability of high temperature coating 110 to act as an EBC, while theincreased porosity of overlying coating region 114 may improve theoverall strain compliance and thermally insulative properties of hightemperature coating 110. The relative thicknesses of sintered coatingregions 114, 116 will vary amongst implementations. However, by way ofexample, innermost sintered coating region 116 may be thinner thanoutermost sintered coating region 114; e.g., in an embodiment, the moredense innermost sintered coating region 116 may constitute between about10% to about 40% of total coating thickness, while the less denseoutermost sintered coating region between about 60% to about 90% oftotal coating thickness. In other embodiments, high temperature coating110 may contain additional layers and/or other variations in porosity,such as those described above in conjunction with FIG. 2.

CONCLUSION

There has thus been provided high temperature coatings well-suited forformation over ceramic turbomachine components including, but notlimited to, silicon-ceramic GTE components. Embodiments of the hightemperature coating are bonded directly to the component bodyeliminating reliance on bond coats. Through the elimination of bondcoats, failure paths involving recession, delamination, and other bondcoat compromise are precluded. Coating manufacturing processes may alsobe eased. A high integrity bond is formed between the high temperaturecoating and the underlying ceramic component body or substrate utilizinga sinter bonding process, by removing surface oxides, and by deterringsurface oxide regrowth until at least the initial stages of sinterbonding. Iterative deposition and firing processes may be performed tocompile the high temperature coating to a desired thickness over theceramic coating body. The high temperature coating may also have acontrolled, non-zero porosity, which, in certain embodiments, may varyin a step-wise fashion through the coating thickness. Such controlledporosities can improve the thermally insulative abilities and straincompliance of the high temperature coating, while preserving coatingtoughness and coating-to-substrate bond integrity. The resultant hightemperature coating may be relatively resistant to penetration ofcombustive byproducts, such as steam, which can otherwise erode theunderlying (e.g., silicon-containing) ceramic substrate body orsubstrate. Although not limited to any particular application, the hightemperature coatings may be particularly well-suited for formation overselected surfaces of ceramic GTE components directly exposed to hightemperature gas flow during GTE operation.

While multiple exemplary embodiments have been presented in theforegoing Detailed Description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing Detailed Description willprovide those skilled in the art with a convenient road map forimplementing an exemplary embodiment of the invention. It beingunderstood that various changes may be made in the function andarrangement of elements described in an exemplary embodiment withoutdeparting from the scope of the invention as set-forth in the appendedClaims.

What is claimed is:
 1. A method for forming a spallation-resistant hightemperature coating over a turbomachine component having a ceramiccomponent body, the method comprising: removing a surface oxide layerfrom the ceramic component body to expose a treated surface of theceramic component body, the ceramic component body comprises asilicon-nitride material and the removing comprises contacting thesurface oxide layer with molten sodium hydroxide (NaOH) over thesilicon-nitride material and selectively removing silica scale and glassphases from the silicon-nitride material to produce a non-planar surfacetopography on the treated surface; depositing a first layer of a coatingprecursor material over the treated surface, the coating precursormaterial infiltrating into the non-planar surface topography, the firstlayer of the coating precursor material having a solids content composedpredominately of at least one rare earth silicate by weight percentage;and heat treating the first layer of the coating precursor material tosinter the solids content and form a first sintered coating layer bondeddirectly to the treated surface of the ceramic component body with amechanical lock between the first sintered coating layer and the treatedsurface.
 2. The method of claim 1 further comprising: depositing asecond layer of the coating precursor material over the first sinteredcoating layer; and heat treating the second layer of the coatingprecursor material to form a second sintered coating layer bonded to thefirst sintered coating layer.
 3. The method of claim 2 furthercomprising: embedding organic particles in the second layer of coatingprecursor material; and during heat treatment of the second layer of thecoating precursor material, thermally decomposing the organic particlesto impart the second sintered coating layer with an increased porosityrelative to the first sintered coating layer.
 4. The method of claim 3further comprising selecting an amount, size, and shape of the organicparticles to impart the second sintered coating layer with a porositybetween about 20 and about 40 percent by volume.
 5. The method of claim3 further comprising forming at least one additional coating layer overthe second sintered coating layer having a porosity less than theporosity of the second sintered coating layer.
 6. The method of claim 1wherein heat treating comprises: performing an organic burnout phaseduring which the first layer of coating precursor material is heated toa first peak temperature within a first time period; and after theorganic burnout phase, performing a sintering phase during which thefirst layer of coating precursor material is heated to a second peaktemperature within a second time period; wherein the first peaktemperature is less than the second peak temperature; and wherein thefirst time period is greater than the second time period.
 7. The methodof claim 6 further comprising, during the sintering phase, heating thefirst layer of coating precursor material at a rate exceeding 300degrees Celsius per minute to attain the second peak temperature.
 8. Themethod of claim 1 further comprising maintaining the ceramic componentbody in non-oxidizing atmospheres for a period of time encompassing thesteps of removing the surface oxide and heat treating the first layer ofthe coating precursor material.
 9. The method of claim 1 whereinremoving comprises imparting the treated surface with a surfaceroughness exceeding 0.1 micron.
 10. The method of claim 1 wherein thesteps of removing, depositing, and heat treating are performed such thatthe first sintered coating layer is bonded directly to and intimatelycontacts the treated surface of the ceramic component body.
 11. Themethod of claim 1 further comprising selecting the solids content of thecoating precursor material to comprise: 80% to 100% at least one rareearth silicate by weight; and 0% to 20% at least one glass sintering aidby weight.
 12. The method of claim 11 further comprising selecting theat least one rare earth silicate to comprise ytterbium disilicate. 13.The method of claim 11 further comprising selecting the at least oneglass sintering aid to comprise 1% to 10%, by weight, of a sinteringglass aid selected from the group consisting of magnesioaluminosilicate,magnesia, and alumina.
 14. A method for forming a high temperaturecoating over a turbomachine component having a silicon-nitride componentbody, the method comprising: removing a surface oxide layer from thesilicon-nitride component body to expose a treated surface of thesilicon-nitride component body, the removing comprises contacting thesurface oxide layer with molten sodium hydroxide (NaOH) over thesilicon-nitride component body and selectively removing silica scale andglass phases from the silicon-nitride component body to produce anon-planar surface topography on the treated surface; building-up asintered coating body over the treated surface of the silicon-nitridecomponent body by iteratively performing the steps of: (i) depositingcoating precursor material layers containing rare earth silicateparticles over the treated surface, and (ii) heat treating the coatingprecursor material layers to sinter the rare earth silicate particlesand form a portion of the sintered coating body, the building-upincluding depositing a first layer of the coating precursor materialover the treated surface and heat treating the first layer of thecoating precursor material to sinter the rare earth silicate particlesand form an innermost layer of the sintered coating body, with amechanical lock formed between the innermost layer of the sinteredcoating body and the treated surface; and imparting the sintered coatingbody with a desired porosity by: (i) adding organic particles to atleast a first layer of the coating precursor material layer included inthe coating precursor material layers and, (ii) thermally decomposingthe organic particles when heat treating the first coating precursormaterial layer.
 15. The method of claim 14 wherein imparting thesintered coating body with the desired porosity further comprisesimparting the sintered coating body with a porosity that varies from aninnermost layer of the sintered coating body to an outermost layer ofthe sintered coating body.
 16. The method of claim 15, wherein theoutermost layer has a porosity that is greater than a porosity of theinnermost layer.
 17. The method of claim 15 further comprising forming atop coating layer over the outermost layer of the sintered coating bodyhaving a porosity less than the porosity of the outermost layer of thesintered coating body, the top coating layer forming an outermost layerof the high temperature coating.